This paper investigates the solid state transformer applied for the HVDC transmission from the offshore windfarm. The 5kVA experimental setup is developed and comparison of 50Hz, 400Hz and 1000Hz operation. Dependency of the loss in the transformer with different core materials to frequency is investigated. Frequency loss dependency of several core materials are compared with experimental measurement. Nano crystaline material called “FINEMET” is the best performance, although amorphous seems to have better cost performance.